A 39-year-old Washington man who has been convicted of three robberies was indicted here yesterday in another armed robbery under a new federal law that imposes a 15-year mandatory minimum sentence on certain repeat offenders.

U.S. Attorney Joseph E. diGenova said the indictment of Horace Graydon Jr. of 36 G St. SW is the first in the District of Columbia under the Armed Career Criminal Act, which Congress passed a year ago.

In December diGenova announced the establishment of a screening procedure to find defendants who might be prosecuted under the law.

The legislation was sponsored by Sen. Arlen Specter (R-Pa.), chairman of the Senate Appropriations subcommittee on the District.

Yesterday, diGenova said it had taken more than nine months to "find a defendant that met the profile" set by the act, which requires three previous convictions for robbery or burglary but does not consider other crimes.

Graydon is charged with robbery in the holdup Aug. 23 of an armored truck outside a downtown Washington bank. The holdup resulted in an exchange of gunfire in front of the National Museum of American Art. No one was injured.

Anthony K. Henry, 25, of 1724 Potomac Ave. SE, and Don A. Shaifer, 24, of 1407 Ames Place NE, also were indicted yesterday in the case and charged with armed robbery, assault, and firearms violations. They and Graydon are being held without bond.